FOND DU LAC, Wis. (AP) -- A Wisconsin man opened fire on police during a six-hour standoff at a house, killing one officer and critically wounding another, authorities said.

Police in Fond du Lac were called to the house around 6:30 a.m. Sunday to investigate a sexual assault. Shortly after they arrived, the suspect started shooting at the officers, a news release said.

SWAT teams surrounded the house during the standoff. Hours later, the suspect, James Cruckson, 30, was found dead inside the house from an apparent self-inflicted gunshot wound, police said. Authorities used armored vehicles and shields to evacuate residents from nearby homes during the standoff.

Police said Cruckson's sister was in the house at the time. It wasn't immediately clear whether she was injured during the standoff.

Deputy Chief Kevin Lemke said Cruckson had prior contacts with police, but he wouldn't give details. Court records show he pleaded no contest and was found guilty of misdemeanor battery in 2009.

Police would not provide any further details about the shootings and said they went to the house while "continuing their investigation into a sexual assault."

Fond du Lac police Officer Craig Birkholz, 28, was shot in the upper chest and killed, police Chief Tony Barthuly said. K-9 Officer Ryan Williams was shot twice and was taken to an area hospital, where he was in critical condition, Barthuly said. Williams' dog, Grendel, also was shot.

A third officer, Zach Schultz, was injured on a stairwell in the house. Police say Schultz was not injured by gunfire.